Method of producing planar grids for vacuum tubes



1957 c. PRUSZYNSKI 2,810,186

METHOD OF PRODUCING PLANAR GRIDS FOR VACUUM TUBES Filed March 22, 195226' Fly 4 INVENTOR (l /[5754 P/Ql/SZYNS/f/ BY ATTORNEY United StatesPatent Office 2,810,186 Patented Oct. 22, 1957 METHOD OF PRGDUCIN GPLANAR GRIDS FOR VACUUM TUBES Chester Pruszynski, Huntington Station, N.Y., assignor to Sylvania Electric Products Inc., a corporation ofMassachusetts Application March 22, 1952, Serial No. 278,090 6 Claims.(Cl. 29-2514 This invention relates to a new and improved method offabricating planar type grid structures for vacuum tubes and toapparatus for production of grid structures by that method.

A form of planar grid, which is commonly used in the art, consists of asmall, washer-like ring frame with grid wires stretched across one faceof the ring parallel to a diameter, and fastened thereto, so as to lieacross the central opening of the washer-like frame, through which theelectrons pass from cathode to anode in the operation of the tube.

it is an object of this invention to provide :an improved process forquantity production of planar grids affording a more satisfactory anduniform product at a re-' duced cost and permitting an increase in thenumber of grid turns per inch that may be applied.

it is a further object of my invention to provide a process that doesnot require the use of engineers or skilled technicians in theproduction of such grids, but which may be carried on by reasonablyskilled labor accustomed to the manufacture of tubes.

In the past, many difficulties have been encountered in manufacturingsuch grids. Among the more troublesome of these may be mentioned:

1) Variation in angular velocity of the wire feed spool.

(2) Limitation on the minimum diameter of wire that could be used, and,therefore, on the grid turns, per inch.

(3) Non-uniformity of the grids produced, resulting in highmanufacturing shrinkage and high production costs.

(4) Loss of tension in the grid wires caused by excessive heating of thewires during the process of attach ing them to the frame.

(5) Loss of tension in the grid wires caused by the grid framecontracting more than the grid wires as the grid cools after brazing.

(6) Flow of solder out over the surface of the grid wires in the activearea of the grid.

The difiiculties arose out of the method heretofore used for winding andbrazing the grids. According to conventional practice, a pair of frameswere placed one on each side of a flat mandrel which rotated at uniformangular velocity during the winding operation. This resulted in theangular velocity of the feed spool varying from zero to some maximumvalue twice during each revolution of the mandrel. This, in turn,required that with each velocity pulse or jerk on the wire, the wire,instead of merely being subjected to enough tension to overcome thedynamic friction of the tensioning device, had to withstand sufficienttension to overcome the-static friction of the tension device andaccelerate the feed spool as well. If the tension device were adjustedso that the wire was nearly at the breaking point during peakacceleration, the average tension in the wire was only a small fractionof the maximum value. This in turn placed a limitation on the number ofturns per inch which could be applied because the finer wire which mustbe used to allow the use of higher turns per inch suffers an evensmaller ratio of average to peak tension. This is due to the fact thatthe frictional and accelerating forces of the wire spool are as high forthe fine wire as for the larger Wires.

Other difliculties arose due to the use of a high meltingpoint solder.It was necessary to use gold, having a melting point of 1063 C. in orderto braze the tungsten or molybdenum wire to the molybdenum frame. Nosolder with a lower melting point satisfactorily wets molybdenum.

Still other difiiculties arose from the fact that the mandrels carryingthe wound grids on each side had to be brazed in a hydrogen #bell jar,one side at a time. Each time the grid was heated to rbrazingtemperature some of the wire tension was lost. Frequently, even thoughthe grids were wound at the maximum tension the Wire would stand Withoutbreaking for more than ten percent of the time, the completed grids hadno tension left in them.

In addition to the shrinkage of grids above described,

costs were high because after completion of one set of grids themandrels had to be individually polished by hand, a very expensiveoperation. All these problems had existed for some time, but until myinvention there was no solution to them.

According to my invention, all these difiiculties are eliminated orgreatly reduced, enabling the grids to be manufactured in quantityproduction at greatly reduced cost and with greatly improved uniformityof product.

An object of the invention resides in the provision of an improvedmethod of brazing or otherwise bonding a plurality of filamentaryelements, such as grid wires, to a supporting member provided with anaperture across which the wires extend. In certain types of vacuum tubesthe grid assembly is fabricated, prior to being mounted within the tube,and consists of a disc of molybdenum or other metal, centrally:apertured, with a plurality of grid wires extending in parallelrelation across the aperture with end portions brazed or otherwisebonded to one surface of the disc, and maintained under tension toprevent sagging when heated. The present invention is primarily directedto improvements in the manufacture of this type of grid structure,although the principles may be employed in the manufacture of similararticles for other intended uses.

The invention further encompasses improvements in the application ofinduction brazing as applied to the manufacture of vacuum tube gridstructures. More specifically such improvements include employment ofradio frequency electrical energy to efi'ect induction flash brazing ofgrid wires to an apertured supporting member in a manner to preventbuckling of the grid wires during the brazing operation and to insuremaintenance of optimum tension in each wire upon subsequent cooling ofthe assembly.

Other objects include improvements in apparatus for carrying out themethod. An important feature of such apparatus resides in theconfinement of heat flow resulting from the brazing operation in amanner to prevent metal flow along the grid wires within the aperturearea of the grid disc, since the presence of such solder is injurious tothe tube in which the grid is used, inasmuch as the solder melts andvaporizes when the grid is mounted in close proximity to the cathode,and in addition may promote the formation of undesired sulfides as aresult of exposure to ordinary industrial atmospheres.

From the general improvement and simplification of methods and apparatusfor manufacture of vacuum tube grid assemblies, in line with the abovestated objectives, and as described in detail below, other applicationswill become apparent to persons skilled in this art without departurefrom the scope of the invention as set forth in the appended claims. Inthe drawings, in which like parts are identified by the same referencenumerals:

Fig. 1 shows in perspective apparatus for winding grid wire over mandrelsupported apertured disc-like grid elements.

Fig. 2 is a front elevational view of apparatus employed in thefabrication of grid assemblies and incorporating principles of theinvention.

Fig. 3 shows in plan an indexable mandrel and associated inductionheating apparatus. Fig. 4 illustrates details of the high frequency coilstructure of Fig. 3 adapted to establish flux density transversely of amandrel such as shown in Figs. 1, 2 and 3.

Fig. 5 shows in plan, a grid structure, the wires of which have beenwound and brazed by the method and apparatus herein described.

Certain electron tubes, such as the so-called planar grid tubes, employflat type grid assemblies which are mounted transversely of the tubeenvelope in spaced relation to the cathode and anode structures. Gridstructures of this type which have heretofore been used, are commonly inthe form of a washer-like centrally apertured disc 10, as shown in Fig.5, with a plurality of grid wires 11 stretched across one face thereofand brazed at 12 and 13 with grid wires 11 extending, under tension,across a central aperture 15 which is aligned, upon assembly of thetube, with the cathode and anode structure. Such grid structures, andthe type of electron tubes in which they are employed, are shown anddescribed in Patent 2,553,580 of Paul Haas, issued May 22, 1951, andassigned to the assignee of the present invention. In that applicationboth grid and anode structures are secured to supporting electrodeswhich extend transversely of and project through the tube envelope insealing engagement therewith. Since the present invention does notrelate to the tube structure per se, further reference is limited to thegrid structure only, and to the improved method and apparatus forfabricating such structures, as above mentioned.

As shown in Figs. l through 4, apparatus for reducing the improvedmethod to practice includes a polygonal work mandrel, genericallydesignated 18, preferably of a material of high heat resistivity, andelectrical insulating properties, such as a commercial product, known asLava, although other materials such as ceramics having these propertiesmay be employed. Mandrel 18 is first rotatably mounted on spindle 19 toa suitable winding device, not shown, for application of a continuousgrid wire in a manner shown in Fig. 1 Discs 10 have been mounted asshown to each polygonal face of mandrel 18 and retained by a ring starclamp 20 to prevent displacement of the discs 10 during the windingoperation. If desired, a pair of such ring star clamps may be employed,one on each side of the mandrel. A feed spool 22 is rotatably mounted ina position to permit feeding of wire 11 about the mandrel 18 and discs10, mounted thereon, as spaced by wire guide device 24, which includes atensioning device pictorially represented by block 25 and a guide meansrepresented by notched block 26.

Polygonal mandrel 18 is preferably as near a cylinder as is practical toprovide a large angle between planes. For example, a hexagonal mandrel,two and one half inches in diameter provides supporting areassufficiently large to carry the grid frames for certain tubes. Such amandrel closely resembles a cylinder as to uniformity of demand fromwire feed spool 22, permitting employment of greater tension ontensioning device 25 than heretofore allowed with more residual tensionresulting in the wire after winding, since the consequent reduction ofvelocity changes insures subtsantially constant tension during thewinding operation.

Prior to the winding operation which is done in a conventional manner,apparent from Fig. 1, grid wire 11 has one end secured to the mandrel,and the opposite end tied thereto after winding. The manner in which thediscs 10 are prepared for soldering, prior to the winding operation, isdescribed below. Upon completion of the winding operation mandrel 18 isremoved from the winding device, spindle 19 removed, and the mandrelmounted by bolt 17 on a support member 26 of a size convenient for benchuse. The particular member shown includes an upright bracket portion 22of reduced thickness, and a laterally extending base plate 24 for rigidsupport on a bench or other supporting surface 25. Smooth glass caneinserts, 27 are positioned within slots 29 at the corners of thepolygonal mandrel to insure uniform application of the grid wire 11during winding, since rough corners have been found to result in ahelical winding of non-uniform spacing. While mandrel 18 as shown is ofhexagonal configuration, as described above, it is understood that theinvention is not limited to any one type of mandrel, nor is it limitedto the use of mandrels generally, since the inventive concept isapplicable to other means of indexing discs 14 together with a uniformlyspread overlay of grid wires 11 to an area of concentrated highfrequency electrical energy, described below, while supporting the discsand wires in a manner. to confine heat flow substantially to localizedareas during the brazing operation. Advantages reside in the employmentof a rotatable mandrel, since the grid wire may quickly be helicallyapplied with a uniformity of spacing and tensioning not otherwise soreadily obtained. Simplicity and low manufacturing costs also favormandrel type indexing over more complicated equipment.

Each work receiving surface of mandrel 13, such as 30, Fig. 1, isprovided with a pair of transverse slots 31 32, so spaced that eachapertured disc 10, when centered on the surface, extends across slots 31and 32, with the area to be brazed, as "shown at 12, Fig. 5 locatedcentrally of each slot. The wires are applied under tension, however,since during tube use, the grid wires heat to a greater extent than doesthe supporting disc, hence sufiicient cold tension must be applied toinsure against sagging of the 7 wires, which would result in changedelectrical characteristics, or even in short circuits to the cathode.With the mandrel indexed to the position shown in Fig. 4, the brazing ofthe grid wire, transversely of the coil formed thereby, is as follows.

A work coil, generically designated 48, and best shown in Figs. 2through 4 is positioned immediately above a selected disc 10 to whichgrid wire 11 is to be brazed. While wire 11 is continuous, hencesingular, the convolutions are cut after both ends are brazed to eachdisc 10, hence will hereinafter be referred to as a plurality of wires11. The function of work coil 40 is to concentrate high frequencyelectrical energy, carried to coil 40 by conductors 41 and 42,-from asuitable source, not shown, the energy concentration being confined to alimited area of disc 10, to the grid wires 11 within that area, and tobonding material, applied to the wires within that area to effect thedesired braze, as described below. Coil 40 consists of a single turnincluding horizontal going and return portions 41 and 42, Figs. 2 and 3.Portions 41 and 42 are closely adjacent having therebetween only anarrow slot or space 50 through which may be dropped a length of solderin wire form, properly to position the solder across the grid wires 11just prior to application of the R. F. energy.

A feature of the present invention resides in the structure of work coil40. As shown in Figs. 2 through 4, the coil preferably is constructed ofrigid stock, such as copper or other electrically conducting material.As shown, the coil material may be rectangular in cross section. Thecoil 40 includes a pair of supporting electrodes 48 and 49, Fig. 2,adapted for connection to a source of high frequency electrical energyand positioned upright as shown in Fig. 4. Upright portions 48 and 49lead into horizontally disposed portions 41 and 42, Figs.

2 and 3, both upright and horizontal portions being closely spaced asshown at 50. The horizontal portions 41 and 42 extend beyond thevertically positioned mandrel 18, Figs. 3 and 4, to be integrally orotherwise joined at 52 to form a closed loop. Through an area of greaterwidth than the mandrel 18 and in registry therewith, the stock ofportions 41 and 42 is milled to form diagonally disposed downwardlytapering portions 44 and 45 as best shown in Fig. 2. The structure justdescribed serves as a work coil for concentrated application of highfrequency electrical energy to grid discs and the grid wires woundthereon to effect brazing of the wire to the grid. It has been foundthat the diagonally disposed portions 44 and 45 of reduced thicknesseffectively concentrate electrical heating energy as appliedtransversely of the mandrel face, within the area immediately adjacentthe solder as shown at 56, Fig. 2. Direct heating of the portions of thedisc running generally parallel to the grid wires is thus avoided,minimizing the differential contraction and expansion experiencedbetween grid wires and disc fabricated of different materials.Undesirable heating of these portions of the disc by conduction from theheated portions may be minimized by reducing the duration of the time ofapplication of high frequency energy to a minimum.

As above mentioned, methods heretofore employed in brazing the gridwires 11 to the disc 10 have resulted in the flow of the brazing solderalong each grid wire 11 into the area defined by aperture 15, toincrease the wire diameter marginally of the aperture or even to fill inthe space between the wires. Such an increase in wire size or alterationof the electron permeable area is highly undesirable, since it changesthe electrical characteristics of the tube and exposes an inferior metalon the grid wires to the electron stream causing difficulties in theintended operation of the tube, such as grid emission. The presentinvention operates to provide a grid disc in which the solder does notflow onto the grid wires.

With the method of brazing taught herein, the mandrel stays cool and itis possible to employ glass cane inserts 27, as above mentioned, at thecorners to provide an extremely smooth winding surface which is highlyresistant to the cutting action of the fine grid wire which may beutilized in the grid. The inserts may easily be replaced whenaccidentally scratched or cut by the grid wires.

For purposes of example, the present invention has been reduced topractice for mass production of grid assemblies, the discs of which areof molybdenum. Since this material cannot be wet by silver solder, whichis commonly employed in the manufacture of electron tube interiorassemblies, the discs are gold plated, with-the gold fused thereto in adry hydrogen furnace. Gold has a melting point of 1945 F. as compared to4750 F. for molybdenum. The grid wires used are of platinum coatedtungsten, 0.0003 inch in diameter, wound over the discs at 896 wires tothe inch. Platinum has a melting point of 3223 F. as compared to 6100 F.for tungsten. The solder used is composed of 60% silver, 30% copper, and10% tin, With a melting point of 1105 R, which is sufficiently high towithstand internal electron tube temperatures in the types with whichthe grid structure is used. This solder will wet both gold and platinumat the melting point of 110S F., but will not wet molybdenum directly.It is used in place of gold because it melts at a lower temperature,thus giving rise to less hot stretching and less differentialcontraction between the discs and the grid wires.

What I claim is:

1. The method of fabricating planar grid structures comprisingconstructing an apertured member of metal having a melting pointsubstantially higher than gold, plating said member with gold,maintaining a plurality of filamentary elements of material having ahigher melting point than gold in mutually spaced and tensionedcondition and in contiguous relation to a face of said apertured memberto extend transversely of the aperture, positioning a solder of lowermelting point than gold transversely of said wires for support therebyabove a solid portion of said apertured member, and subjecting saidsolder, wires, and an immediate portion of said apertured member tolocal, concentrated high frequency electrical energy to braze said wiresto said member, said immediate portion being adjacent but spaced fromsaid aperture.

2. In apparatus for winding planar grids, a polygonal work supportingmandrel of high heat resistivity material the faces of which areprovided with a pair of spaced transverse channels to provide isolatedwork contacting surfaces at either end and centrally thereof to reduceheat flow therebetween, means for mounting said mandrel for step by steprotation through a vertical plane, and means for concentrating radiofrequency electrical energy within the space immediately above onechannel.

3. The apparatus of claim 2, wherein said energy concentrating meanscomprises a work coil having a portion thereof extending the length ofone of said channels midway of the projected area thereof and adjacentthe mandrel surface, whereby electrical energy may be applied inproximity to a planate work piece positioned on said surfaces to extendacross said one channel.

4. The method of forming a vacuum tube grid assembly consisting of thesteps of forming a disc of molybdenum, providing said disc with acentral aperture, maintaining said disc against a transversely channeledwork supporting member of high heat resistivity in a manner to extendacross the channeled portion thereof, extending tungsten wire across theexposed face of said disc in contiguous engagement with the surfacethereof, applying silver solder to a portion of said wire at a point ofengagement with said disc and within the projected area of said channel,and subjecting said solder and the immediate portions of said wire anddisc to radio frequency energy to heat said solder to the melting pointand effect an amalgamized bond between the solder, wire, and disc.

5. An induction heating electrode assembly comprising a pair of closelyspaced electrically conducting elements joined at one end and adaptedfor connection to a source of high frequency electrical energy, saidelectrodes having aligned portions of reduced thickness diagonallydisposed in V-configuration for concentration of electrical energy alongthe apex thereof.

6. The method of forming a planar grid assembly consisting of the stepsof mounting an apertured grid disc on a mandrel and supporting parts ofsaid disc to which solder flow is not desired against surfaces of saidmandrel, extending wires across the exposed face of said disc in contactwith portions of the surface thereof on either side of an aperture,applying solder at points of contact of said wires and said surfaceportions where said disc is unsupported, and subjecting said solder andcontiguous regions of said wire and said disc to a local field of radiofrequency energy to heat said solder to the melting point and effect abond between the solder, wire, and disc.

References Cited in the file of this patent UNITED STATES PATENTS1,937,097 Simon Nov. 7, 1933 2,194,551 Holman Mar. 26, 1940 2,314,865Biewirth Mar. 30, 1943 2,314,875 Gillespie Mar. 30, 1943 2,439,517Johnson Apr. 13, 1948 2,500,355 Haas Mar. 14, 1950 2,655,589 SorensonOct. 13, 1953 FOREIGN PATENTS 124,909 Australia July 31, 1947 600,257Great Britain Apr. 5, 1948

